Load Commutated Inverter (LCI) Synchronous Motor Drives using a thyristor inverter bridge do not need forced commutation means, because automatic thyristor turn-off is achieved with a synchronous motor as the load, if it has a leading phase angle with respect to the load voltage. For a given load, increasing sufficiently the field will produce such leading power factor. See, for instance:
"The Synchronous Machine as a Self-Controlled Converter-Fed Motor" by Dieter Kollensperger in Siemens Review XXXV (1968) No. 5, pp. 195-201;
U.S. Pat. No. 4,713,743 of Dec. 15, 1987 (Alberto Abbondanti).
With an induction motor, however, this possibility no longer exists. The load power factor is lagging for all machine excitation levels. Therefore, specific circuitry must be used to allow a leading phase angle to take place, thereby providing natural commutation of the inverter bridge. To this effect, a general solution is to add a large capacitor bank in parallel with the motor, so that the lagging load power factor be overcompensated, the net result being that leading VAR's are supplied to the composite load. Accordingly, the resultant leading power factor angle will insure natural commutation of the inverter poles. In such case, the motor drive is referred to as a Load Commutated Inverter Induction Machine (LCI/IM) drive. Thus, when the power factor of the induction motor has been overcompensated to produce a leading load power factor to a current source, in principle the induction motor behaves much like a synchronous motor as far as current source inverter (CSI) is concerned, for a significant speed range.
Traditional Current Source Control (CSI) method, used where the inverter bridge includes thyristors and forced-commutation is applied for an induction motor drive, consists in regulating the DC-Link current magnitude in order to achieve the desired motor voltage. Motor speed is regulated by imposing the desired switching upon the inverter. The desired motor voltage is maintained proportional to the imposed frequency. This prior art approach, however, has revealed problems of stability, particularly encountered at low speed. One solution to this problem, as typified by U.S. Pat. No. 4,602,198 of Herbert W. Weiss et al, has been to use in an LCI system a VAR generator in order to control the VAR supply. This is an expensive and complex approach to the problem.
The present invention bears uon the problem of stability encountered with a Load Commutated Inverter Induction Machine (LCI/IM) drive, i.e. one where the induction motor has a capacitor bank in parallel. The invention is based on the observation that, but for certain deviations which are minimal, the firing angle of the inverter in a CSI control loop is practically equal to the power factor angle on the output lines from the inverter to the load, i.e. the induction motor.